This invention relates to systems and method of measuring the stroke volume of a heart, and more particularly to such systems and methods based on measurement of intracardiac impedance.
In recent years, a number of attempts have been made to measure left or right ventricular stroke volume (SV) by intracardiac electrical impedance change (.DELTA.Z). Examples of the foregoing are disclosed in the following U.S. patents:
______________________________________ Pat. No. Inventor Issue Date ______________________________________ 4,898,176 Petre Feb. 6, 1990 4,840,182 Carlson Jun. 20, 1989 4,823,797 Heinze et al. Apr. 25, 1989 4,721,115 Owens Jan. 26, 1988 4,686,987 Salo et al. Aug. 18, 1987 4,674,518 Salo Jun. 23, 1987 4,535,774 Olson Aug. 20, 1985 4,303,075 Heilman et al. Dec. 1, 1981 4,291,699 Geddes et al. Sep. 29, 1981 ______________________________________
Impedance catheters of the type disclosed is such patents typically have pairs of electrodes on their distal ends in a bipolar or tetrapolar configuration. Early work on the use of catheter-based electrodes was performed by Palmer at Baylor University and reported in 1970 in a PhD thesis entitled "Continuous Measurement of Stroke Volume by Electrical Impedance in the Unanesthetized Animal." Palmer employed the bipolar method, which involves injection of a constant current between two electrodes disposed within a chamber of a heart, e.g., one of the ventricles, with the resultant voltage between the electrodes being measured as an indication of the impedance of the blood in the portion of the ventricle between the electrodes. The tetrapolar configuration also involves establishing a current flow between two source electrodes disposed in the ventricle, but voltage is measured between an inner pair of electrodes disposed between the two source electrodes. The principle in either case is that the current flowing between the catheter electrodes is confined largely to the ventricle in which the catheter is located because the resistivity of the ventricular wall and septum is appreciably higher than that of blood. As a first approximation, the ventricle may be modeled as a cylinder with a sensing electrode at each end. The impedance between the electrodes is inversely proportional to the volume of blood between the sensing electrodes and directly proportional to the blood resistivity. At end diastole the ventricular volume is the highest during the cardiac cycle and the impedance is lowest, whereas at end systole the ventricular volume is the lowest and the impedance is highest, the impedance change (.DELTA.Z) being related to stroke volume (SV).
However, with a bipolar or tetrapolar electrode system, the current distribution between the source electrodes is not uniform. A large portion of the current hugs the catheter and relatively little spreads to intercept the free wall, as shown in FIG. 1 for bipolar electrodes la and lb in the right ventricle (RV). It is difficult to obtain adequate current spread by increasing the electrode spacing because of artifacts produced by the tricuspid valve and papillary muscles. Moreover, if the electrodes come close to the septum or ventricular walls, the relationship between .DELTA.Z and SV is different, making it difficult to obtain a .DELTA.Z that bears a constant relationship to SV.
A significant amount of effort has been put into improving the accuracy of the bipolar method. Baan et al., as reported in "Continuous Stroke Volume and Cardiac Output from Intraventricular Dimensions Obtained with Impedance Catheter," Cardiovasc. Res. 15:328-334 (1981), and "Continuous Measurement of Left Ventricular Volume in Animals and Humans by Conductance Catheter," Circulation 70:812-823 (1984), modeled the ventricle as a column of stacked cylinders each defined by a pair of sensing electrodes, and added the individual cylinder volumes to obtain total ventricular volume. Similar techniques are described in the above-referenced patents to Salo, Owens, Carlson and Petre, along with proposals for solution of various problems associated with the method. Salo sought to solve the problem of nonuniform current density by using extrapolation to compute an impedance value equivalent to what would be measured with drive electrodes spaced an infinite distance apart, from which distance the electric field lines between the electrodes would theoretically be straight and parallel. Regarding the number of required electrodes with this technique, Owens indicates that as few as four would be sufficient for measurement of relative stroke volume, although accompanying thermal dilution measurements would be important in such cases, but that ten electrodes would be more appropriate for assessment of absolute stroke volume. Owens and Peter both disclose stiffening members intended to space the electrodes away from endocardial tissue. Carlson introduced an algorithm designed to eliminate overestimation of stroke volume caused by parallel conductance of the heart wall and surrounding tissue. The algorithm requires measurements at two different frequencies and solution of simultaneous equations at the two frequencies to cancel out parallel conductance terms.
We believe that a catheter-borne, monopolar electrode offers significant advantages over all known bipolar, tripolar and tetrapolar electrode configurations, including, among other things, a more constant relationship with stroke volume. In contrast to the bipolar and tetrapolar methods described above, the monopolar method involves use of a single electrode at the site of the impedance change, and an indifferent or reference electrode distant from the site of activity. The conductive case of a pacemaker implanted in a patient's chest may serve as the reference electrode. A constant current is injected between the active and reference electrodes and the resultant voltage is measured between the same two electrodes. Theoretically, current spreads radially from the active electrode, as shown in FIG. 2 for a single monopolar electrode 2, such that there is increased opportunity for the current to intercept the free wall, thereby providing a more constant relationship between .DELTA.Z and SV. A study in our laboratory has shown that the impedance measured in the foregoing manner is insensitive to the location of a reference electrode on the chest.
Monopolar electrode configurations appear in the above-referenced Heinze et al. patent in the context of stroke volume measurement for pacemaker control, in U.S. Pat. Nos. 4,697,591 and 4,790,318 to Leckholm et al. and Elmqvist et al., respectively, in the context of pacemakers controlled by a respiratory signal, and in U.S. Pat. No. 4,805,621 to Heinze et al. in the context of a pacemaker controlled by an impedance signal component directly related to metabolism and not influenced by respiration or stroke volume. Davis et al., in an article entitled "Driving Electrode Configurations in Cardiac Conductance Volumetry," in IEEE Eno. Med. & Med. Biol. Soc. 10th Ann. Conf. 757-758 (1988), concluded from theoretical studies based on a spherical model of the ventricle that a configuration having a monopole current source at the origin and a sink at infinity, with the potential sensed at an arbitrary distance from the source, has advantages over bipolar configurations for measuring stroke volume. Mentioned as a possibility in the article is a volumetric sensor with sensing electrodes placed spherically about a monopole source within the ventricle.
A number of disadvantages remain with all available configurations and techniques for measurement of stroke volume based on intracardiac impedance. Large artifacts can be encountered with any catheter-electrode system when a current-injecting electrode comes close to the ventricular walls, valves or septum. Furthermore, even with a monopolar electrode configuration, the relationship between .DELTA.Z and SV is not linear over a wide range of stroke volumes, although a linear relationship appears to hold for a smaller range of volumes. Electrode position in the ventricle also appears to be important for reasons unrelated to artifacts. There is not necessarily any one position that is optimal for stroke volume measurements in all patients. An optimal position for one patient may be unsuitable for another, particularly considering that the ventricles often do not contract uniformly in diseased hearts, which are most in need of proper diagnosis and therapy and which are most likely to be controlled by a pacemaker.
There is a related need, particularly with the advent of exercise-responsive pacemakers, to establish a pacing rate or rates best suited for a patient's individual physical condition and his anticipated range of activities. Cardiac output defines the pumping capability of the ventricles, and a primary determinant of cardiac output is heart rate. Accordingly, there is an increasing need to know cardiac output at different pacing rates at rest as well as during exercise. At present there is limited information on the subject of cardiac output versus pacing rate in pacemaker patients. Some studies conducted on normal subjects at rest indicate that cardiac output changes little before decreasing with an increase in pacing rate. However, in patients with impaired ventricles, a slight increase in pacing rate may cause a decrease in cardiac output. A survey of studies on this subject is included in a paper by Geddes et al. entitled "The Exercise-Responsive Cardiac Pacemaker," published in IEEE Transactions on Biomedical Enoineerino BME 31(12):763-770 (1984). This paper describes a three-phase relationship which was found to exist between cardiac output and pacing rate and includes a graphical illustration of the relationship, which is also illustrated in this application, in FIG. 6A, along with an additional curve showing the possibility of cardiac output remaining constant in Phase II of the resting state. Further information on this subject may be found in a paper by Fearnot et al., entitled "Control of Pacing Rate Using Venous Blood Temperature," on pages 69-72 of the 39th ACEMB Proceedings, Sept. 1986, and in an article by Wessale et al. in the May, 1988 issue of PACE. entitled "Cardiac Output Versus Pacing Rate at Rest and With Exercise in Dogs With AV Block."
Cardiac output depends on the heart rate, which is controlled by the pacemaker, and the stroke volume, which relies on the pumping capability of the ventricles. The pacing rate which results in maximum cardiac output for a given exercise level is the optimal pacing rate for that exercise level. Optimum pacing rates need to be determined for rest and exercise when a sensor-driven pacemaker is implanted. Unfortunately, the optimum pacing rate varies from patient to patient, because, among other reasons, stroke volume is dependent on the contractile status of the myocardium, and because patients have varying degrees of underlying cardiac disease. There is a need for an objective method of choosing optimum pacing rates, and it is believed that such an objective method requires a convenient way to obtain data on a particular patient's cardiac output over a range of pacing rates for each selected workload.
Another application of impedance-based sensors of stroke volume is in the detection of ventricular fibrillation. Such applications are likely to become widespread in the near future, particularly in automatic implantable cardioverter-defibrillators, which can save the life of a patient at risk of recurrent ventricular fibrillation by administering a defibrillating shock when needed. During ventricular fibrillation, stroke volume drops to essentially zero, and one would expect a corresponding drop in the pulsatile impedance change detected in the ventricle. This principle has been employed in a defibrillator designed for automatic actuation only when the mechanical activity and electrical activity of the ventricle both indicate a need for defibrillation. This defibrillator, described in U.S. Pat. No. 4,291,699 to Geddes et al., used a bipolar catheter-borne electrode pair to detect the cardiac electrogram and also to detect the pulsatile impedance change in the ventricle. The defibrillator delivered a shock only when both electrocardiographic and impedance criteria for fibrillation were met, reducing the probability of applying an inappropriate shock.
In a paper entitled "Optimal Spacing of Right Ventricular Bipolar Catheter Electrodes for Detecting Cardiac Pumping by an Automatic Implantable Defibrillator," Med. Instrum. 14:27 (1980), Tacker et al., using large-area, catheter-borne electrodes, evaluated five different spacings between bipolar electrodes and concluded that a 5 mm separation identified ventricular fibrillation more reliably than wider electrode separation. Our recent research in this area suggests, to the contrary, that the greater the separation between electrodes, the more rapidly the pulsatile impedance amplitude ratio decreases at the onset of fibrillation. Relatively closely spaced electrodes appear to be more susceptible to artifacts which may adversely affect the ability to reliably detect loss of stroke volume during fibrillation.